The present invention relates to the field of sources of polarized light as well as methods of providing polarized light, more particularly for the polarization of light for use e.g. in light valve devices such as for example LCD or LCOS projectors.
A liquid crystal device (LCD) or liquid crystal on semiconductor (LCOS) device may be used in various applications, such as for example displays (e.g. displays in laptop computers, watches and calculators) and projection systems for projecting information or images onto a distant screen.
An LCD or LCOS projector basically comprises a lamp for generating light, illumination optics for capturing that light and transferring it to one or a plurality of LCD or LCOS devices comprising light valves, and a projection lens which images the illuminated LCD or LCOS device(s) on a screen. A basic property of both LCD and LCOS devices is that they work with polarized light, more particularly with linearly polarized light. Polarized light is used together with the properties of the liquid crystal elements to selectively transmit/reflect or absorb light to produce a pattern of light and dark pixels, thus creating a desired image. Because almost all lamps used in projectors generate non-polarized light, which is light comprising at least two polarization directions, this light has to be polarized in an optical system either before it reaches the LCD or LCOS device(s) or in the device itself. This may be done by only selecting that part of the light which has a desired polarization direction, e.g. using a polarising filter, which method, however, leads to a loss of light output for the projector.
To overcome this problem, typical LCD or LCOS projectors use a polarization recuperation system that splits the unpolarized light from the lamp into two light beams which each have one of the two independent polarization directions. Each light beam with a different polarization direction follows a different optical path. The two independently polarized states for example can be two orthogonal linear polarizations or as another example a left and right circular polarization. The light with one of the two polarization directions is then converted into the other polarization state by a retarder or a polarization rotator. Thereafter it is cast on the LCD or LCOS devices via another optical path. This system avoids throwing away the light having the unwanted polarization state and therefore almost doubles the efficiency of the illumination optics.
One prior art embodiment of such a polarization recuperation system is a lightpipe combined with a polarising beam splitter (PBS) system that is introduced in the optical path, as explained in U.S. Pat. No. 5,884,991. Light from the lamp is sent into the entrance of the lightpipe by using a reflector and possibly also some extra condensor lens or lenses. Inside the lightpipe the light is reflected a number of times on the side surfaces, and is homogenised when it reaches the end of the lightpipe. Essentially the exit of the lightpipe is imaged onto the LCD or LCOS devices by a set of lenses. The lightpipe is used to make the illumination of the LCD or LCOS device(s) more uniform. A PBS is provided in the optical path, after the lightpipe. A PBS is an optical element designed to split light into two linearly orthogonal polarization directions, the s and p polarizations. Light with one polarization direction is reflected by the PBS, light with the other polarization direction is transmitted. According to the embodiment described in U.S. Pat. No. 5,884,991, on one of the two exit faces of the PBS a half wave retarder is provided, which converts light with one linear polarization direction into light with the other linear polarization direction.
According to another embodiment of a prior art polarization recuperation system, as described in WO 02/17000, the polarization recovery system is built in a lightpipe. The lightpipe obtained is highly complex and has more than 6 surfaces. This complicates the manufacturing of the lightpipe and the alignment in the projector, and has a negative effect on the homogenising property of the lightpipe.
Furthermore, it is a disadvantage of both prior art solutions described that the finally formed polarized light beam occupies an area which is larger than the area occupied by the original light beam, which means that less light intensity reaches every light valve of the LCD or LCOS device illuminated by means of the lightpipe or that additional optical components are required to focus the beam onto a smaller area.
It is an object of the present invention to make effective use of the light available in an optical system and method, while using a light polarizer in the optical system.
It is a further object of the present invention to provide an optical system and method which produces a concentrate beam of polarized light having a beam diameter comparable with that of input light to the system.
The above objective is accomplished by a device and a method according to the present invention.
The present invention provides an optical polarization recuperation device for producing a substantially polarized light beam from an unpolarized light beam comprising first and second polarizations, the device comprising:
an optical circuit having an entrance area for receiving incoming light rays, and an exit area for allowing light rays to exit the optical circuit,
a reflective polarizer means adjacent the exit area of the optical circuit, for directing light rays having the first polarization into a first light path and for directing light rays which have the second polarization into a second light path in the optical circuit,
a mirror at the entrance area of the optical circuit, with a means for reflecting those light rays following the second light path which fall onto it, and
a quarter wave or an odd multiple of quarter wave retarder means through which at least a part of the light in the optical circuit and which is to be emitted from the optical circuit passes,
wherein light following the first and the second light path goes through the same part of the optical circuit.
According to the present invention, the first and the second light path go through the same part of the optical circuit.
The optical circuit may be a lightpipe.
According to an embodiment of the present invention, the retarder means is provided in both the first and second light path.
According to an embodiment of the present invention, the retarder means may be placed at the exit area of the optical circuit. According to another embodiment, the retarder means may be placed at the entrance side of the optical circuit.
The retarder means may be a quarter wavelength or an odd multiple of a quarter wave retarder or a number of retarders which co-operate together to form a quarter wave or an odd multiple of a quarter wave retarder. For example, the retarder means may be formed from two eighth wavelength retarders.
The retarder means may be a broadband retarder.
According to an embodiment of the present invention, the reflective polarizer means may be a polarising beam splitter. According to another embodiment of the present invention, the reflective polarizer means may be a wiregrid polarizer.
An optical polarization recuperation system device to the present invention may furthermore comprise a depolarizer means for depolarising light. When the light rays are emitted from a light source having electrodes, the depolarizer means may comprise plasma between the electrodes of the light source. A depolarizer means may be located anywhere else in the second light path as well.
The present invention also provides optical polarization recuperation device for producing a substantially polarized light beam from an unpolarized light beam comprising first and second polarizations, the device comprising:
an optical circuit having an entrance area for receiving incoming light rays, and an exit area for allowing light rays to exit the optical circuit, a reflective polarizer means at the exit area of the optical circuit, for directing light rays having the first polarization along a first light path and for directing into the optical circuit light rays which have the second polarization, the light rays having the second polarization following a second light path in the optical circuit,
a mirror at the entrance area of the optical circuit having means for reflecting those light rays following the second light path which fall onto it, and
a depolarizer means for depolarizing light, the depolarizing means being at least in the second light path.
The depolarizer means may comprise a birefringent material in the cross-section of the light pipe, more specifically in case the optical circuit is a lightpipe, the depolarizer means may comprise a birefringent material on any inner surface of the lightpipe.
When the light rays are emitted from a light source having electrodes, the depolarizer means may comprise a plasma between the electrodes of the light source.
The optical circuit may be a light pipe. For example the lightpipe may be a hollow structure with internally mirrored surfaces. Alternatively, the lightpipe may be a solid rod in which reflections to the side surfaces are based on internal reflection of light with a large incident angle, such as a glass rod for example. The lightpipe may have a constant cross-section or it may be tapered towards the exit area.
The reflective polarizer means comprises a cholesteric liquid polarizer.
The present invention furthermore comprises a method for recuperating polarized light said method comprising the steps of:
providing a beam of unpolarized light rays comprising first and second polarizations, the beam of unpolarized light rays following a first light path,
impinging said beam of unpolarized light on a reflective polarizer means, so as to direct light rays having the first polarization along the first light path and to direct light rays having the second polarization along a second light path,
reflecting the reflected light rays following the second light path,
changing the orientation of the polarization of the light rays in both the first and the second light path.
The present invention furthermore comprises an LCD projector or an LCOS projector comprising any of the above optical polarization recuperation devices.
The present invention simplifies the optical set-up of a projector by implementing polarization recuperation inside a single lightpipe, without adding complexity to the shape thereof. The lightpipe can be made out of 4 side surfaces, an entrance and an exit interface, as conventional lightpipes. No restriction on the present invention is contemplated dependent upon the type of lightpipe used.
It is an advantage of the optical polarization recuperation system and method of the present invention that the efficiency of the projector is increased by using a polarization recuperation system, while preserving the compactness of the projector, and while avoiding the use of polarization recovery systems outside the lightpipe.
These and other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.